Known types of digital tuners are of single conversion or double conversion types. In the single conversion type, the incoming signal is converted to a fixed intermediate frequency (IF) which is sufficiently low for the received signals to be digitised and then demodulated in the digital domain. Double conversion tuners convert the incoming signal to a first intermediate frequency signal, which is then converted to a second intermediate frequency signal sufficiently low to permit digitisation and demodulation.
Another known type of tuner is of the direct conversion or zero IF type, in which the incoming signal is converted directly to the baseband signal in a single frequency conversion step and is then digitised and demodulated. Zero IF tuners are generally used where the tuning range of the tuner is narrow relative to the frequencies of the incoming signals. For example, in DBS (Direct Broadcast Satellite) systems, the tuning range or received frequency range is from 950 to 2150 MHz.
Zero IF tuners are not readily suitable for applications where the tuning range is relatively large compared with the received signal frequencies. For example, in a cable or terrestrial tuner, the tuning range is typically 50 to 900 MHz. When the tuner is tuned to one of the lower channels of such a large tuning range, there are a significant number of channels at harmonics of the tuned frequency. The mixer function within a zero IF tuner is generally highly non-linear so that there is the substantial risk of channels at frequencies which are integer multiples of the tuned frequency also being converted to the zero IF and hence of causing substantial interference, possibly rendering the technique unusable.
GB 2 316 249 discloses a digital radio receiver of the double conversion type comprising an input bandpass filter between the aerial input and the mixer of a first frequency changer of the upconverter type. The output of the first mixer is supplied via a bandpass filter at the first high intermediate frequency to the mixer of a second frequency changer of the downconverter type. The downconverter converts the high first intermediate frequency signals to a relatively low second intermediate frequency such that the selected signal has a bandwidth in the frequency range from 0 to 30 MHz. The second mixer is required to have high isolation characteristics in order to prevent breakthrough of the downconverter local oscillator signal to the input of the mixer of the upconverter. The frequencies of the local oscillators of the upconverter and downconverter are said to be outside the range of frequency of the broadcast signal.
GB 2 192 104 and WO 98/35544 disclose double conversion tuners in which the second frequency changer is of the quadrature type.